A multilayered material sheet comprising a consolidated stack of unidirectional monolayers of drawn polymer is known from EP 1627719 A1. This publication discloses a multilayered material sheet comprising a plurality of unidirectional monolayers consisting of ultra high molecular weight polyethylene and essentially devoid of bonding matrices, whereby the draw direction of two subsequent monolayers in the stack differs. A monolayer of the multilayered material disclosed in EP 1627719 A1 is produced by positioning a plurality of tapes of ultra high molecular weight polyethylene adjacent to each other whereby adjacently positioned tapes overlap at least partly along their side edges. Without the overlap the known multilayered material cannot be produced. Also, in order to obtain good antiballistic properties, the material sheet of EP 1627719 A1 makes use exclusively of ultrahigh molecular weight polyethylene, essentially free from bonding matrices.
Although the multilayered material sheet according to EP 1627719 A1 shows a satisfactory ballistic performance, this performance can be improved further.
The object of the present invention is to provide a multilayered material sheet having at least similar antiballistic properties as the known material, and which sheet can be easily produced.
This object is achieved according to the invention by providing a multilayered material sheet comprising a consolidated stack of unidirectional monolayers of drawn polymer, whereby the draw direction of two subsequent monolayers in the stack differs, whereby at least one monolayer comprises at least one unidirectional tape of the drawn polymer, each tape comprises longitudinal edges, whereby the monolayer is free from overlaps, or free of an area of elevated thickness adjacent to and along the substantial length of the longitudinal edges. Preferably, the monolayer is free from overlaps, or free of the area of elevated thickness adjacent to and along at least 50%, 60%, 70%, 80%, 90% or 95% of the length of the longitudinal edges of the at least one unidirectional tape. Most preferably, the monolayer is free from overlaps, or free of an area of elevated thickness along and adjacent to the entire length of the longitudinal edges of the at least one unidirectional tape.
The formation of monolayers which are free from overlaps or excessive levels of binders enable the monolayers to be more readily stacked and compressed into a multilayered material sheet with uniform areal density resulting in a more homogeneous antiballistic performance across the multilayered material sheet.
In one embodiment of the present invention, this objective is achieved by a multilayered material sheet and a process to produce such multilayered material sheet, the sheet comprising a consolidated stack of unidirectional monolayers of drawn polymer, whereby the draw direction of two subsequent monolayers in the stack differs, whereby at least one monolayer comprises a plurality of unidirectional tapes of the drawn polymer, aligned in the same direction, and whereby adjacent tapes do not overlap.
It turns out that a multilayer material sheet according to the invention, i.e. a sheet wherein areas of elevated thickness adjacent to and along the longitudinal edges of the unidirectional tapes e.g. longitudinal overlaps of tapes or tapes extensively bound together with overlapping binding material are substantially absent, not only improves the antiballistic properties of the sheet, but does so to an unexpectedly high extent. Preferably, the monolayer is free of an area of elevated thickness which extends along (rather than transverses across) and adjacent to the longitudinal edges of the at least one unidirectional tape. The occurrence of areas of elevated thickness which extends along and adjacent to the longitudinal edges of the unidirectional tape or unidirectional tapes are more difficult to form a homogeneous consolidated stack compared to areas of elevated thickness attributed to the transverse overlapping of unidirectional tapes, such as that observed when the tapes are aligned such that they form a woven structure.